Beamformers are utilized with arrays of electromagnetic radiating elements and of sonic radiating elements, the latter referred to as transducers, for combining signals of the radiating elements to produce beams of electromagnetic energy and beams of sonic energy. The term beam is used both for a beam of transmitted radiant energy as well as for radiant energy received from a particular direction since the transmitting and receiving radiation patterns of an array of radiating elements are identical. Beamformers employ linear circuits for summing together the signals of the respective radiating elements and for imparting selective delays, or sometimes only phase shifts, to signals of the respective radiating elements. The selection of specific values of time delay is based on the direction of the desired beam relative to the array.
In some situations, the signals of the radiating elements are sampled repetitively to produce sequences of signal samples from each of the radiating elements. The sequences of samples are then transmitted to the beamformer which forms one or more beams as is desired. One important example is the case of a sonar array which is towed by a ship, in which case the beamformer is located on board the ship and separated from the array by a long cable which carries signals between the sonar transducers of the array and the beamformer. Typically, the signals of the transducers are sampled and are then multiplexed to permit their coupling via a single communication channel to the beamformer.
A problem arises in that the accuracy of the beam pattern depends on the sample rate, except in a special circumstance such as the broadside beam of a line array for a plane wave wherein the arrangement of the radiating elements within the array permits the occurrences of the signal samples of the respective radiating elements to be synchronized with a wavefront of the radiation. As is seen in an article entitled "Digital Array Phasing" by Victor C. Anderson which appears in the Journal of the Acoustical Society of America on Pages 867-870 in July 1960, beams can be constructed in a limited number of directions for which the samples selected from a history of the samples from the respective transducers lie on a wavefront of a wave in the respective directions. Such beams may be referred to as synchronous beams since the requisite delays of the respective samples are in synchronism with the times of arrival of the wavefront at the respective transducers. For all other beam directions, a compromise must be made to select a beam sample which has occurred either prior to or subsequent to the desired instant of time. As a result of this compromise, the radiation pattern of the beam is also compromised with the resultant degradation in the quality of the radiation pattern depending on the sample rates. Thus, a higher sample rate produces a smaller quantization error in the time delay of the samples selected for producing the beam.
The need for higher sample rates produces other problems. The cable which serves as the communication channel between the transducers of a towed array and the beamformer is of limited bandwidth, this limiting the rate at which signal samples can be transmitted. In systems employing the conversion of analog-to-digital signals, it has been the practice to time share a single analog-to-digital converter among several transducers with a selector switch being utilized for selectively coupling the signals of the respective transducers to the analog-to-digital converter. However, due to a limitation in the speed at which such converters can operate, the higher sampling rate decreases the number of transducers which may be sharing a single one of the converters so that a greater number of such converters is required. The problem is further compounded in large arrays having many radiating elements for producing a larger number of highly directive beams of radiation as is done with systems measuring angles to a high accuracy and a high resolution. The need for higher sampling rates also increases system complexity by requiring a higher storage capacity to the memory which stores the signal samples. The foregoing problems may also exist in the case of an electromagnetic array wherein a limited bandwidth communication channel is provided between the array and a beamformer. The ensuing description of the invention will be directed to a sonar situation for convenience in explaining the invention, it being understood that the use of the term transducer applies equally well to the radiating elements of the electromagnetic situation.